1. Field of the Invention
The present invention relates to rotation control of a recording medium (disk) in a data storage apparatus such as a hard disk apparatus (drive).
2. Background Art
With a data storage apparatus (hereinafter referred to as “disk drive”) that records data on a recording medium that rotates, such as a hard disk drive, it is vital to achieve highly precise rotation control. In particular, as the capacity of recording media increases and data is stored at a higher density, strong measures are necessary against oscillation of the direction of rotation due to external factors.
Conventionally, in the case of rotation control using a 3-phase DC motor, rotation is controlled by measuring the zero-crossing time at which the counter electromotive force induced in the phase intersects the reference voltage. For example, in the case of a motor that outputs a 6-cycle voltage waveform during one circuit (one revolution), a possible method is to have the controller measure the time of each cycle and compare it with a target time, and obtain a feedback value.
Since the counter electromotive force waveform is determined by the arrangement of magnets in the motor, the times of the six cycles in the voltage waveform are not the same even if the motor is rotating at constant speed. Thus, conventionally, measures are taken to reduce the controller gain and prevent an excessive change in the feedback values in each cycle so that the motor rotates with good precision even if there is dispersion of the six cycle times in the voltage waveform. For this reason, the open loop function zero-crossing frequency is approximately 1/15th of the rotation frequency of the motor.
FIG. 6 is a drawing showing the configuration of the rotation control apparatus of a spindle motor (DC motor) in a conventional disk drive.
As shown in FIG. 6, an output wave resulting from adding the influence R of field dispersion based on the position of magnets (hereinafter referred to simply as “magnet position dispersion”) to output Y of a spindle motor 61 is input to a controller 62. The output of the controller 62 is then fed-back to the spindle motor 61.
Taking time as output, transfer function P(z) of the spindle system (spindle motor 61) in FIG. 6 is non-linear, but can be approximated by means of the equation in Numeric Reference 2 below.                               P          ⁡                      (            z            )                          =                              -                          1              2                                ⁢                                    T              3                        θ                    ⁢                                    (                              z                +                1                            )                                      z              ⁡                              (                                  z                  -                  1                                )                                              ⁢                                    k              t                        J                                              [                  Numeric          ⁢                                           ⁢          Reference          ⁢                                           ⁢          2                ]            
In the case of spindle motor 61 in which a 6-cycle voltage waveform is output during one cycle (one revolution), T is rotation time/6 and θ is 2π/6. Also, kr is the torque constant and J is inertia.
At this time, the influence of magnet positions on the rotation of the spindle motor 61 is expressed by the equations in Numeric Reference 3 below.                               Y          =                                    F              Phase                        ⁢            R                          ⁢                                  ⁢                              F            Phase                    =                      PH                          1              -              PH                                                          [                  Numeric          ⁢                                           ⁢          Reference          ⁢                                           ⁢          3                ]            
In the equations in Numeric Reference 3 above, transfer function P(z) of the spindle motor 61 is denoted by P. H is transfer function H(z) of the controller 62, and is expressed by the equation in Numeric Reference 4 below.                               H          ⁡                      (            z            )                          =                              k            p                    +                                    k              1                                      z              -              1                                                          [                  Numeric          ⁢                                           ⁢          Reference          ⁢                                           ⁢          4                ]            
It is assumed that the spindle motor 61 rotates at 4200 rpm and that 6 times are sampled in one revolution. At this time, the base frequency of magnet position influence R is 70 Hz. It is therefore necessary to select a controller 62 that will enable the 70 Hz component to be adequately suppressed. For example, in PI control, a controller 62 is designed for approximately −30 dB at 70 Hz, the magnet position influence R base frequency. FIG. 7 is a drawing showing the error function (Rejection function) in this case.
At this time, the following apply to the controller 62:un=kp(tn−ttarget)+kiInIn+1=In+tn−ttarget ki=11.7kp=1170where un is the spindle current, tn is the measured time, and ttarget is the target time.[Problems to be Solved by the Invention]
If controller gain is reduced in order to absorb magnet position dispersion in the spindle motor, feedback values from the controller do not change even if the voltage waveform of the motor is influenced by disturbance, and it is difficult to eliminate the influence of disturbance.